The failure of synthetic polymeric medical devices - Durability and Reliability of Medical Polymers - page 182

unaffected product or component is ideal, this is not always possible. Visual
inspection aims to identify the following product features as a minimum:

∑ overall dimensions;
∑ distortion in dimensions;
∑ fit with matching parts;
∑

surface quality;

∑ traces of wear; and

∑ identity marks.

The search for key details does not stop at the fracture surface, however.
Cracks which have not grown to completion are one objective of the search.
such subcritical cracks provide evidence of the way the component has
been loaded in service, and might show why failure has occurred in the
first place.

9.2.1 Microscopy
Optical microscopy of the fragments is essential, although it is often
surprising how much information can be revealed by visual inspection with an
eyeglass. Optical microscopy of reflected light is the easier to use, allowing
manipulation of the sample under a wide variety of lighting conditions.
Fibreoptic sources are particularly good for shadowing oblique surfaces
so that fine details become visible through high contrast. Transmission
is possible through transparent polymers such as silicone rubber, while
birefringent polymers such as polycarbonate can show how the sample was
made. scanning electron microscopy (seM) is the more powerful inspection
method because the resolution is much higher and it gives a good three-
dimensional view of surfaces. it also has the added advantage of providing
elemental composition of the sample by analyzing the emitted x-rays from
the polymer by electron irradiation.

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9.2.2 Material analysis
Material analysis follows if any chemical change in the polymer used in
the device or implant is suspected and it is often adopted to identify or
confirm the polymer in question. Such methods include differential scanning
calorimetry DsC and spectroscopy, of which the most important is Fourier
transform infra-red (FTiR) spectroscopy. Calorimetry provides data on the
thermal properties of the polymer such as the melting point T m , the glass
transition temperature T g , and the presence of additives. it is important to
compare the sample with known standards. Figure 9.1 shows the melting
points of various types of polyethylene (PE), allowing characterization
of unknown samples. spectroscopy depends on detecting absorption of a